The beneficial effects of employing ammonia as a working refrigerant in vapor compression refrigeration systems has been known since the late 19th century. Those skilled in the art have recognized that ammonia has many advantages when utilized as a refrigerant. As a first matter, it has a high critical temperature; and secondly, a low triple point temperature which allows it to be applied over a wide range of applications. These applications include air conditioning applications where the air is maintained at temperatures greater than about 45 degrees F., to low temperature refrigeration applications where the air temperature must be maintained at temperatures at or below −40 degrees F. Ammonia has a latent heat of vaporization which is considered high and which reduces the mass flow required for any given refrigeration load. The direct result of this latent heat of vaporization is that for a given refrigeration load, the resulting liquid line sizes are relatively small. Still further, other thermodynamic and thermophysical properties of ammonia result in good heat transfer coefficients. This results in efficient and compact heat exchanger designs being employed in various applications.
Ammonia is also considered to be an environmentally friendly, or “green” refrigerant since it occurs in nature and has no known capacity for depleting ozone in the atmosphere. It further has no apparent global warming potential. Those skilled in the art recognize that ammonia is used widely in a number of industry segments and in various applications. Ammonia is relatively easy to produce and is low in cost as compared to other halo-carbon refrigerants now being employed.
While ammonia has been known for a long period of time and has many advantages, it also has some disadvantages which have detracted from its usefulness. Chief among its shortcomings is that ammonia is toxic in high concentrations; is an irritant in low concentrations; and further has a very pungent order when released. Still further, ammonia is flammable in a narrow range of concentrations with air. Another serious shortcoming with ammonia is that ammonia has a significant affinity for water. Ammonia readily reacts with any water which may inadvertently get introduced to a refrigeration system and thereafter holds the water tightly in solution. In the prior art ammonia refrigeration systems utilized heretofore, water has always been considered a contaminant. It has been known that it is extremely difficult to keep water out of a prior art ammonia refrigeration system. Unfortunately, even in small amounts, an aqueous ammonia refrigerant can significantly increase the boiling point of the refrigerant mixture resulting in reduced refrigeration system performance, and increased operating costs. Typically, the presence of only a small amount of water in the prior art ammonia refrigeration system, employed heretofore, will typically cause an expansion valve control function to fail. If this failure is left unintended the ever increasing concentration of water in the refrigerant increases the boiling point of the ammonia-water concentration until the expansion valve controller is no longer able to sense the correct amount of superheat in any resulting refrigerant vapor. If left uncorrected, this same ammonia-water refrigerant can ultimately irreparably damage a compressor employed with the same refrigeration system.
Heretofore, industrial ammonia evaporators employed with prior art refrigeration systems have been typically fed with liquid refrigerant in one of several ways. These ways have included gravity flooding; liquid overfeed; and direct or dry expansion. With respect to both prior art gravity flooding, and liquid overfeed ammonia refrigeration systems, these systems require relatively large inventories of liquid ammonia refrigerant circulating between various vessels, and the evaporators employed with these systems. On the other hand, direct expansion ammonia refrigeration systems operate with the smallest amount of ammonia refrigerant inventory possible. In view of the aforementioned advantages, and disadvantages, of ammonia refrigerant discussed, above, direct expansion ammonia refrigeration systems have become quite attractive, at a number of different levels, for the owners and operators of these same systems. For example, the ability to operate with a low ammonia refrigerant charge in a refrigeration system is desirable because, as a first matter, this reduces the cost of manufacturing these same systems by allowing for the elimination of pressure vessels, pumps and the reduction of liquid line sizes. Secondly, direct expansion ammonia refrigeration systems are attractive because of their reduced risk of fire or explosion. Still further, they present reduced risks should an ammonia leak occur. Additionally, because of these reduced risks of system damage or worker injury because of the smaller amount of ammonia refrigerant being used, owners of such systems may experience a lower insurance rate and further reduced EPA and OSHA health and safety requirements for installing and operating such systems.
Not with standing these many advantages, an efficient and highly effective direct expansion ammonia refrigeration system has proved elusive to designers. Prior art direct expansion ammonia refrigeration systems have continued to suffer from poor evaporator performance caused by undesirable two phase flow patterns of the ammonia refrigerant in the evaporator tubes, from malfunctioning thermostatic expansion valves, and the consequent damage to compressors resulting from the return of ammonia-water solutions to the compressors caused by the effects noted, above. Consequently, owners and operators of prior art ammonia refrigeration systems have had to live, heretofore, with larger ammonia refrigerant inventories associated with gravity flooded and pump recirculated arrangements as will be described in greater detail hereinafter.